Diffusion metallic coating method

ABSTRACT

A METALLIC ARTICLE SURFACE IS PROVIDED WITH A DIFFUSION METALLIC COATING OF IMPROVED OXIDATION AND SULFIDATION RESISTANCE THROUGH A VAPOR DIFFUSION METALLIC METHOD CONDUCTED IN A NON-OXIDIZING ATMOSPHERE AS A RESULT OF MAINTAINING THE ARTICLE SURFACE AND A COATING MATERIAL WHICH SUPPLIES THE METALLIC COATING IN SPACED APART RELATIONSHIP WHILE HEATING BOTH THE SURFACE AND THE COATING MATERIAL TO PERFORM VAPOR DIFFUSION COATING. THE ARTICLE IS FIRST PROVIDED WITH A COATING OF LOOSELY ADHERENT OXIDE PARTICLES WHICH A ENTRAPPED IN THE DIFFUSION COATING.

United States Patent 3,598,638 DIFFUSION METALLIC COATING METHOD David J. Levine, Cincinnati, Ohio, assignor to General Electric Company No Drawing. Filed Nov. 29, 1968, Ser. No. 780,199 Int. Cl. C23c 9/02 US. Cl. 117107.2P 3 Claims ABSTRACT OF THE DISCLOSURE A metallic article surface is provided with a diffusion metallic coating of improved oxidation and sulfidation resistance through a vapor diffusion metallic method conducted in a non-oxidizing atmosphere as a result of maintaining the article surface and a coating material which supplies the metallic coating in spaced apart relationship while heating both the surface and the coating material to perform vapor diffusion coating. The article is first provided with a coating of loosely adherent oxide particles which are entrapped in the diffusion coating.

The useful life of certain high temperature operating metallic articles under corrosive or oxidizing conditions can be increased through the application of a protective coating to the metallic article surface. For example, such conditions might be found in the hot portions of power producing apparatus as gas turbine engines. Frequently, the dimensions and shape of thin walled ducts such as combustors must be maintained accurately before and after coating. Therefore, one class of coating which has been applied to such articles is generally known as the metallic diffusion coating applied by a high temperature vapor deposition method.

This type of process depends upon the diffusion of metallic elements into-the surface of the article to be protected. Therefore, it is conducted at a relatively high temperature, for example in excess of 1400 F. and generally in the range of 16002100 F. Also, it is carried out in a non-oxidizing atmosphere such as in an inert gas, a reducing gas such as hydrogen, a self-generated atmosphere, or in a vacuum.

Known high temperature metallic diffusion coating methods depend upon intimate contact between the article surface and the coating material from which evolves the metallic element to be interdiffused with the article surface. Most widely used is the pack cementation method in which the article is immersed in a powdered or particulate mixture from which the coating elements are supplied during processing. When relatively large articles are to be processed by this method, large packs of material must be used. As a result, longer heating cycles are required. In addition, the pressure of larger packs at high temperatures tend to distort thin walled members. In addition, because of the physical contact between an article surface and the material supplying the coating element, problems have resulted from the plugging of small holes and cavities in the articles being coated. Furthermore, there have been problems in removing pack material adhering to the coated surface after processing.

It is a principal object of the present invention to provide an improved vapor diffusion metallic coating method which does not depend upon physical contact between the article surface to be coated and the material supplying the coating element.

Another object is to provide such an improved method particularly for use with a superalloy surface based on one of the elements Fe, Ni and Co.

These and other objects and advantages will be more clearly recognized from the following detailed description "ice and examples which are typical of the practice of the present invention within the scope of the appended claims.

It has been recognized that by maintaining the article surface during a vapor diffusion metallic method in spaced apart relationship with the coating supply material, the problems relating to the plugging of small holes and post method cleaning are obviated. Also, unexpectedly, a coating of improved oxidation and sulfidation resistance results. This is particularly true with coatings resulting from practice of the invention described by co-pending application Ser. No. 693,691 filed Dec. 14, 1967, now Pat. No. 3,540,878, and assigned to the assignee of the present invention.

Described in that co-pending application is a ternary alloy which can be used in a multi-component particulate mixture employed in a metallic diffusion type coating method. The alloy consists essentially of, by weight, about 70% Ti, 20-48% Al and 0.59% combined carbon. It has a dispersion of Ti AlC complex carbide in a matrix of Ti or Al or their alloys. As a component of the particulate mixture used in a metallic diffusion type coating method, it is mixed with about 01-10 weight percent, based on the total mixture, of a halide salt activator which will react with a metallic element in the ternary alloy to form a halide of such element under sufficient conditions of time and temperature in a non-oxidizing atmosphere. As described in that co-pending application, preferred are chlorides and fluorides of ammonium and of the alkali metals of Group I-A of the Periodic Table of Elements. Specifically preferred is about 0.l2% of a halide selected from NaF, KF, NH Cl and NH F.

Metallic diffusion type coating methods are conducted at temperatures of about 1400 F. or above and particularly about 2000 F. Therefore the particulate mixture generally includes, along with the ternary alloy and halide salt activator, an inert filler such as powdered A1 0 which comprises about 1080 weight percent of the particulate mixture. The filler inhibits sintering of the powdered ternary alloy. Thus the particulate mixture described by the above identified co-pending application consists essentially of, by weight, about 2090% of the described ternary alloy, about 10-80% of an inert filler material which will not react with other components of the mixture during use of the mixture and about 0.1l0% of the halide salt activator.

During evaluation of the present invention, a series of high temperature superalloys, particularly those based on nickel and cobalt were used as specimen surfaces. The composition of certain of such superalloys are shown in the following Table I.

The specimens tested included both wrought and cast forms of the alloys in flat panels or paddles as well as in airfoil configurations. In general, the specimens were about /2 inch wide and from 2-4 inches in length. The thickness, depending on the test, varied from about 0.06" to about 0.2". In order to compare the effect of the method of the present invention with that of known vapor diffusion coating methods, longitudinal holes of about 0.05" diameter were placed in specimens. Prior to coating, all specimens were prepared by removing the sharp corners and throughly cleaning.

One form of the above described ternary alloy used in the evaluation of the present invention consisted essentially of, by weight, about 61% Ti, about 34% Al and about 4% combined carbon. A particulate mixture of about 40 weight percent of this ternary alloy powder with about 60 weight percent of powdered alumina filler in which was included about 0.2 weight percent ammonium fluoride activator was used as the coating supply material in the evaluation of the method of the present invention. Neither the alumina filler material nor the halide activator are deposited. The halide activator forms a halide vapor of an element in the metallic portion of the particulate mixture, for example, aluminum. The halide then is thermally decomposed upon contact with the surface of the article. The metallic element of the halide forms an intermetallic with elements such as the base elements nickel or cobalt and the halide is regenerated.

Specimens compared in the evaluation of the present invention were placed in a container with a non-oxidizing atmosphere, in this example, hydrogen. Then they were heated in the range of 1600-2100" F. for 1-4 hours, for example at about 1950 F. for 3-4 hours. Those specimens prepared according to the method of the present invention were suspended in the container in spaced apart relationship and out of physical contact with the particulate mixture coating supply material. Other specimens were placed in physical contact with the particulate mixt-ure prior to heating, for example by immersing them in the mixture.

After processing, a series of specimens were examined in the as-coated condition and then oxidation tested by heating in air at 1800 F. for 2500 hours. Measurements of the outer coating layer, the diffusion layer and the total coating thickness before and after testing are shown in the following Table II for a variety of specimens. Some of the specimens were immersed in the particulate pack (method A) and some were suspended above the pack according to the method of the present invention (method B).

TABLE II applied in accordance with the method described in copending application Ser. No. 780,177, filed concurrently with this application in the size range of about 120 microns.

Metallographic examination of such specimens after application of the diffusion coating showed the oxide particles, which are confined to the outer portion of the coating, to be more uniformly dispersed. Furthermore, such oxides were found to be present in greater concentrations when specimens were spaced from the particulate mixture according to the method of the present invention, than when placed in contact with, such as being immersed in, the pack mixture. For example, in a given period of processing time, of which about 3 hours at about 1950 F. is a specific example, between two and three times the volume percent of entrapped oxides were observed in the coating prepared according to the method of the present invention compared with specimens immersed in the particulate pack mixture.

One series of specimens including the oxide entrapped type of coating was prepared with some specimens out of contact with the pack in accordance with the present invention (method B) and some immersed in the pack (method A). The specimens were placed in a hot corrosion testing apparatus cycled between 1650 and 1725 -F. Heat was generated from combustion of a natural gas/ air mixture. At the same time, 100 parts per million of an aqueous sulfidizing corrodant containing about 9% NaCl and about 10% Na SO was injected toward the specimens. At the end of 75 hours, the specimens were examined metallographically.

Although both types of coatings protected the base material during the 75 hour test, the difference in coating behavior under such hot corrosion conditions was striking. As shown by photomicrographs, there was little evidence of hot corrosion attack on the specimens prepared Coating thickness (mils) It is to be noted that the thickness of the outer coating layer applied according to the method of the present invention is relatively stable. This shows the stability of such layer in inhibiting diffusion in either direction through the layer. The overall enhanced stability through the practice of method B according to the present invention is shown further by the fact that the total coating thickness remained generally constant. Furthermore, the ability of method B to coat the inside portions of longitudinal holes placed in the specimens was at least equal to method A.

All problems with respect to hole plugging were eliminated through the practice of the method B of the present invention. In addition, because the specimens of method B were spaced apart from the particulate pack mixture, there was no problem with respect to the removal of portions of the particulate mixture adhering to the surface. However, some particles from the pack remained tightly adherent to the surface of those specimens in physical contact with the pack mixture.

A series of specimens was prepared as described in the example above except that an interim coating of a powder mixture, by weight, of A1 0 and 50% TiO first was applied to the surface. Such interim coating was about a 10 mil thick layer of loosely adherent, non-fused oxides in accordance with the method of the present invention: spaced apart from the particulate mixture. The diffusion zone remained intact. In contrast, some areas of the method A inpack coated specimens suflered moderate hot corrosion attack in the outer coating layer.

Oxidation tests were conducted at 1800 F. for 2500 hours in air on duplicate specimens to those described above. Metallographic examination revealed that although there was no evidence of coating attack, both the coating outer layer and the diffusion zone doubled in thickness in the specimens prepared by the in-pack method A. The thickness of the coating prepared in accordance with the method B of the present invention remained substantially constant. Thus the coating applied by the method of the present invention is significantly more stable and inhibits diffusion through it both of oxidizing or sulfidizing elements surrounding it as well as of article surface base metal elements with which it is in contact.

Thus by maintaining an article surface to which a metallic coating is to be dilfused by a vapor diffusion metallic method, in spaced apart relationship with a source of the metallic coating elements, a significantly improved diffusion metallic coating is obtained. Such coating is more stable under oxidizing and sulfidizing conditions. Ftuthermore, such practice eliminates the problem of that material adhering to the article surface or entering or plugging holes or channels in the article. In addition, a lesser amount of coating material is needed. As a result, large packs which require longer heating cycles and which tend to distort immersed articles as a result of thermal stresses can be eliminated through practice of this invention.

What is claimed is:

1. An improved vapor deposition metallic coating method for a metallic article surface based on an element selected from the group consisting of Fe, Ni and Co, comprising the steps of:

applying to the article surface an interim vapor permeable coating of loosely adherent, non-fused oxide particles of about 1-20 microns in size and thermally stable during vapor deposition;

placing the article surface in a container in the presence of a coating supply material including a halide and a particulate ternary alloy consisting essentially of, by weight, 5070% Ti, 20-48% Al and 0.5-9% combined C, the alloy including a dispersion of Ti AlC complex carbide in a matrix selected from the group consisting of Ti, Al and their alloys, a metallic portion of which ternary alloy Will form a halide vapor of at least one metallic element of the portion upon heating with the halide;

providing the container with a non-oxidizing atmosphere; and then maintaining the article surface and the coating supply material in spaced apart relationship one from the other; while at the same time,

heating the article surface and the coating material at about 16002100 F. for about 1-4 hours (a) to convert at least one metallic element of the portion into a halide vapor in the non-oxidizing atmosphere,

(b) to entrap by the metallic element at least a portion of the applied oxide particles and to deposit the metallic element on the article surface, and

(c) to interdiffuse the element with the article surface.

2. The method of claim 1 in which the coating supply material is included in a particulate mixture consisting essentially of, by Weight:

20-90% of the ternary alloy;

10-80% of an inert filler which will not react with other components of the particulate mixture during use of the mixture; and

01-10% of a halide salt activator selected from the group consisting of chlorides and fluorides of ammonium and of the alkali metals of Group I-A of the Periodic Table of Elements.

3. The method of claim 2 in which:

the metallic article surface is based on Ni; and

the activator in the particulate mixture is 0.12% of of a halide selected from the group consisting of NaF, KF, NH Cl and NH F.

References Cited UNITED STATES PATENTS 3,108,013 10/1963 Pao Jen Chao et al. 1l7107.2 3,157,532 11/1964 Galmiche 117107.2X 3,276,903 10/1966 Galmiche 117107.2 3,415,672 12/1968 Levistein et al. 117107.2X

ALFRED L. LEAV ITT, Primary Examiner W. E. BALL, Assistant Examiner U.S. Cl. X.R. 117107.2R 

